linux-kernel - Re: [PATCH 1/2] drm/vc4: hdmi: Enforce the minimum rate at runtime

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Date:   Mon, 14 Nov 2022 14:06:08 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     Stefan Wahren <stefan.wahren@...e.com>
Cc:     David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Emma Anholt <emma@...olt.net>, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        Marc Kleine-Budde <mkl@...gutronix.de>
Subject: Re: [PATCH 1/2] drm/vc4: hdmi: Enforce the minimum rate at
 runtime_resume

Hi Stefan,

On Mon, Nov 14, 2022 at 01:48:14AM +0100, Stefan Wahren wrote:
> Am 11.11.22 um 22:08 schrieb Stefan Wahren:
> > Hi Maxime,
> > 
> > Am 29.09.22 um 11:21 schrieb Maxime Ripard:
> > > This is a revert of commit fd5894fa2413 ("drm/vc4: hdmi: Remove clock
> > > rate initialization"), with the code slightly moved around.
> > > 
> > > It turns out that we can't downright remove that code from the driver,
> > > since the Pi0-3 and Pi4 are in different cases, and it only works for
> > > the Pi4.
> > > 
> > > Indeed, the commit mentioned above was relying on the RaspberryPi
> > > firmware clocks driver to initialize the rate if it wasn't done by the
> > > firmware. However, the Pi0-3 are using the clk-bcm2835 clock driver that
> > > wasn't doing this initialization. We therefore end up with the clock not
> > > being assigned a rate, and the CPU stalling when trying to access a
> > > register.
> > > 
> > > We can't move that initialization in the clk-bcm2835 driver, since the
> > > HSM clock we depend on is actually part of the HDMI power domain, so any
> > > rate setup is only valid when the power domain is enabled. Thus, we
> > > reinstated the minimum rate setup at runtime_suspend, which should
> > > address both issues.
> > > 
> > > Link: https://lore.kernel.org/dri-devel/20220922145448.w3xfywkn5ecak2et@pengutronix.de/
> > > Fixes: fd5894fa2413 ("drm/vc4: hdmi: Remove clock rate initialization")
> > > Reported-by: Marc Kleine-Budde <mkl@...gutronix.de>
> > > Signed-off-by: Maxime Ripard <maxime@...no.tech>
> > > ---
> > >   drivers/gpu/drm/vc4/vc4_hdmi.c | 9 +++++++++
> > >   1 file changed, 9 insertions(+)
> > > 
> > > diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c
> > > b/drivers/gpu/drm/vc4/vc4_hdmi.c
> > > index 199bc398817f..2e28fe16ed5e 100644
> > > --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
> > > +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
> > > @@ -2891,6 +2891,15 @@ static int vc4_hdmi_runtime_resume(struct
> > > device *dev)
> > >       u32 __maybe_unused value;
> > >       int ret;
> > >   +    /*
> > > +     * The HSM clock is in the HDMI power domain, so we need to set
> > > +     * its frequency while the power domain is active so that it
> > > +     * keeps its rate.
> > > +     */
> > > +    ret = clk_set_min_rate(vc4_hdmi->hsm_clock, HSM_MIN_CLOCK_FREQ);
> > > +    if (ret)
> > > +        return ret;
> > > +
> > 
> > unfortunately this breaks X on Raspberry Pi 4 in Linux 6.0.5
> > (multi_v7_defconfig + LPAE). Today i saw this report [1] and bisected
> > the issue down to this patch. Shame on me that i only tested this patch
> > with Rpi 3B+ :-(
>
> Looks like "drm/vc4: hdmi: Fix HSM clock too low on Pi4" addresses this
> issue ...

Yes, indeed. The fix should be on its way to -stable already

Maxime

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